Another issue is that Wolverine's healing factor keeps him young, it doesn't make him young, if that makes sense...

There is a difference.

The difference is that the times that Wolverine has lost his healing factor you don't see him just turn old and turn to dust. No, he keeps the age he is, just that he is now aging at a regular rate. Same in reverse... If you gave him back his healing factor he would start to age slowly again, but be at the age he is.

So the point is that Yashida shouldn't have turned young. If he had an ailment then this could fix that, but not turn him young.